The desulfurization of flue gas, particularly flue gas from power plants and industrial processes, has been the subject of considerable study. Air quality laws, both at the federal and state level, have set increasingly stringent emissions standards, especially for such well known pollutants as sulfur oxides. Because coal and oil-fired electric power generating plants can discharge large quantities of sulfur oxides as combustion by-products, much effort has focused on the desulfurization of these flue gases to reduce power plant sulfur dioxide emissions to permissible levels.
Substantial effort has been expended to develop a flue gas desulfurization process which removes substantially all of the sulfur dioxide from the flue gas. Although existing desulfurization process chemistries have achieved high sulfur dioxide removal, these processes have also created other environmental problems, most notably the disposal of the large quantities of solids produced by the most widely used flue gas desulfurization processes. However, available flue gas desulfurization processes also do not operate as reliably or efficiently or cost effectively as might be desired to remove sulfur oxides because the chemistry on which these processes are based requires the addition of expensive chemicals and produces scaling and equipment erosion, which contributes to high maintenance costs.
Most of the existing flue gas desulfurization (FGD) systems employ a suspended solids-containing slurry to contact the flue gas in a scrubber or absorber to remove sulfur dioxide from the flue gas. U.S. Pat. Nos. 3,632,306 and 4,294,807 exemplify sulfur dioxide removal processes wherein an aqueous slurry containing suspended solids is used to scrub sulfur dioxide from waste gas. Slurries such as these contain lime or limestone and the precipitates from reaction of the lime or limestone with sulfur dioxide, mainly calcium sulfite and/or calcium sulfate. The presence of the suspended solids in the scrubbing solution does not initially interfere with removal of the sulfur dioxide pollutants. However, these scrubbing solution solids block and plug FGD system components by solids deposition or scale formation and erode FGD system equipment. The use of a solids-containing slurry as a scrubbing medium may also produce abrasion on the pumps, piping nozzles and other system components, leading to costly equipment repairs and replacement. As a result, FGD systems using suspended solids-containing scrubbing solutions or slurries have not been as reliable as might be desired. Additionally, high liquid to gas ratios are required because such systems have little liquid phase buffering capacity. Moreover, packing cannot be added to the absorber without scaling or plugging of the packing. Consequently, these scrubber systems are relatively expensive to operate, in part because of frequent occurrences of abrasion and mist eliminator plugging, which results in high maintenance costs.
The prior art has proposed methods for removing sulfur dioxide from waste gases using clear wash liquid to eliminate the foregoing problems. The most widely used clear liquor scrubbing process is the dual alkali process, which uses a sodium sulfite clear liquor scrubbing solution. The solution is recirculated in the absorber, and a bleed stream is split off for regeneration in an external reactor. Lime or limestone is mixed with this bleed stream to precipitate the absorbed sulfur dioxide, the salts are separated from the liquor, and the clarified liquor is returned to the absorber for removal of more sulfur dioxide. Although these dual alkali systems have a history of lower capital and maintenance costs for the scrubber system and do not tend to exhibit scaling problems, the high sodium sulfite concentrations required to provide the liquid phase alkalinity results in high operating costs. The high concentration of sulfite and bisulfite in the typical dual alkali sodium buffering system means that only a small change in calcium sulfite relative saturation occurs across the scrubber as sulfur dioxide is removed from flue gas. Sodium is lost from the system in the dewatered filter cake and must be made up with soda ash, which is relatively expensive. Moreover, these absorber systems are designed to require a high concentration of buffer. The high buffer concentration causes high sodium make-up requirements, and the high sulfite concentration makes it impossible to operate at scrubbing pHs below about 5.8 because the high sulfite concentration in the buffer causes a high sulfur dioxide vapor pressure which reduces the scrubbing efficiency. In addition, dual alkali technology directs only a relatively low volume scrubber bleed stream to the reactor and clarifier. This requires high setting rates to reduce the size and cost of the clarifier.
U.S. Pat. Nos. 4,080,428 and 4,222,993 describe other methods for removing noxious contaminants, including sulfur dioxide, from waste gases wherein the contaminated gas is scrubbed or washed with solids-free or clarified liquid. In the process disclosed in U.S. Pat. No. 4,080,428, solids are removed from the wash liquid before it contacts the waste gas, and the pH of the wash liquid is carefully controlled to enhance oxidation and favor the formation of calcium sulfate. Acids, including carboxylic acids, are added to the wash liquid to control the pH and favor the formation of calcium bisulfite, which is readily oxidized to calcium sulfate. The method described in this patent requires a large quantity of wash water to achieve the desired results and, consequently, requires the consumption of a significant amount of energy.
The sulfur dioxide removal method of U.S. Pat. No. 4,222,993 requires a smaller volume of wash liquid than that required by U.S. Pat. Nos. 4,080,428. 4,222,993 discloses that this result can be achieved by the addition to the wash liquid of both an inorganic acid and an organic acid. A polybasic or multibasic carboxylic acid is disclosed to be most effective in reducing the volume of wash liquid required for effective scrubbing. The process of this patent is also conducted under conditions that promote the oxidation of calcium and sulfur-containing compounds to calcium sulfate. Although this is an effective desulfurization process, the calcium sulfate dihydrate or gypsum product formed tends to be deposited as chemical scale on the internal structures of the absorber and the piping so that the efficiency of the process decreases and the maintenance costs increase. Encrustation is a significant problem with this process because of the absence of suspended solids and, hence, seed crystals in the recirculated liquor. In addition, the high calcium ion concentration taught by this patent to enhance sulfur dioxide removal does not avoid but promotes encrustation and scaling.
Flue gas desulfurization processes conducted under inhibited oxidation conditions such as the flue gas desulfurization process disclosed in U.S. Pat. No. 5,266,285 have little difficulty with scaling because a slurry scrubbing system is used. This type of system provides seed crystals, which reduces the relative saturation of calcium sulfite, the precipitated species of sulfur dioxide. Scaling is also avoided in this system by maintaining a sodium sulfite buffering system with high concentrations of sulfite and sulfate. The pick-up in sulfite across the scrubber as sulfur dioxide is removed from the flue gas results in a very small increase in the total sulfite concentration. Consequently, there is very little or no increase in calcium sulfite relative saturation and little or no tendency to form scale in the scrubber in this type of slurry system.
The prior art desulfurization processes employing essentially solids-free or clear scrubbing liquid may effectively remove sulfur dioxide from waste gases under enhanced oxidation conditions that promote the formation of calcium sulfate. However, these processes are not particularly cost-effective because of their scrubbing solution make-up, energy consumption and equipment replacement requirements. Moreover, there is a great tendency for the formation of scale or encrustation in the system structural components. The prior art, therefore, fails to disclose a flue gas desulfurization process conducted under inhibited oxidation conditions that uses a clear, substantially solids-free scrubbing liquid and an organic acid buffer in conjunction with a chemistry which operates at an optimum pH to promote the efficient absorption of sulfur dioxide from waste gases and substantially eliminates the likelihood of scale formation by controlling the relative saturation of calcium sulfite. The prior art further fails to disclose a small volume reactor for a flue gas desulfurization process which improves particle size distribution and dewatering characteristics.
Consequently, a need exists for an efficient and effective desulfurization process and reactor wherein sulfur dioxide is scrubbed from waste gas with a clear, substantially solids-free organic acid buffer-containing scrubbing liquid under inhibited oxidation conditions and an optimum pH which controls the calcium sulfite relative saturation to avoid scaling and encrustation to produce easily dewatered calcium sulfite solids.